1. Field of the Invention
The present invention relates to a process for producing an alkali metal hydroxide. More particularly, it relates to a process for producing an alkali metal hydroxide by electrolyzing an alkali metal chloride at low cell voltage by a diaphragm method using a cation exchange membrane.
2. Description of the Prior Arts
It has been proposed to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride by a diaphragm method wherein an ion-exchange membrane is used instead of using asbestos as a diaphragm so as to obtain an alkali metal hydroxide having high purity and high concentration.
On the other hand, it has been proposed to save energy in the world. From the viewpoint, it has been required to minimize a cell voltage in such technology.
Various methods have been proposed to decrease a cell voltage. It has been proposed to use an electrolytic cell equipped with a gas and liquid permeable anode or cathode bonded on a surface of a fluorinated cation exchange membrane as U.S. Ser. No. 724,968.
In order to minimize an electric resistance caused by an electrolyte or an electric resistance caused by bubbles of hydrogen or chlorine (which have been considered to be difficult for eliminating them), such system is effective as a system for electrolyzing it at lower cell voltage than that of the conventional system.
The anode or the cathode is brought into contact with an ion-exchange membrane in the system. Therefore, the electrode is gas-permeable so as to easily remove the gas formed by the electrolysis from the electrode. That is, the electrode is made of a porous substrate (layer).
The inventors have proposed to produce an alkali metal hydroxide by an electrolysis of an alkali metal chloride at a low voltage by selecting an average pore size and a porosity of the cathode in each desired range. That is, the inventors have found that an alkali metal hydroxide is obtained by an electrolysis of an aqueous solution of an alkali metal chloride at a cell voltage 0.2 to 0.5 V lower than that of the conventional process in stable by using a porous cathode having an average pore size of 0.01 to 1,000.mu. preferably 0.1 to 500.mu. and a porosity of 20 to 95% preferably 25 to 90% bonded on a surface of a cation exchange membrane.
Various substrates and methods can be considered for preparing such cathode having said properties. According to the inventors experiments, it has been found that the porous cathode having said desired properties can be easily obtained without any special manner.